The invention relates to a differential output stage for electronic equipment, for example a telephone subscriber card or a modem in a telephone exchange.
A conventional differential output stage for electronic equipment includes a center-tapped transformer. An AC voltage is applied to the primary winding of the transformer. A DC power supply voltage is applied to the center-tap of the secondary winding. The main terminals of the secondary winding are connected by a line to the terminal which requires a remote power feed. A transformer has the drawbacks of being heavy, bulky and costly and of having a very limited bandwidth. These drawbacks are a particular problem for remote power feeding an HDSL (High bit rate Digital Subscriber Line) modem because a modem of this kind requires 6 W of power, i.e. a power supply current of 60 mA at a voltage of 100 V. This leads to increasing the size of the transformer to prevent it becoming saturated.
European Patent Application EP 0 449 200 (ALCATEL) describes a transformerless differential output stage. It has two inputs, two outputs and two channels each connecting one input to one output. Each channel includes a first differential amplifier for adding to the input voltage of the channel concerned a feedback voltage from the other channel to the channel concerned and a second differential amplifier for subtracting the output voltage of the channel concerned from the output voltage of the first differential amplifier. The output voltage of the second differential amplifier constitutes the feedback voltage from the channel concerned to the other channel.
The above differential stage is perfectly suitable if there is no need for remote power feeding of a terminal by means of a DC current superimposed on the variable current constituting the signal to be transmitted. On the other hand, it is not suitable for remote power feeding a terminal such as a conventional telephone.
The object of the invention is to propose a transformerless differential stage which is capable of remote power feeding a terminal.
The invention provides a differential output stage electronic equipment, said output stage including two channels and each channel connecting an input to an output and including first means for adding to the input voltage of the channel concerned a feedback voltage from the other channel to the channel concerned and second means supplying said feedback voltage from the channel concerned to the other channel, wherein the second means for supplying the feedback voltage from the other channel to the channel concerned include means for supplying a feedback voltage which is a function only of the AC component of the output voltage of the first means of the channel concerned and wherein said output stage further includes respective means in each channel for adding a DC voltage to the output voltage of that channel.
The above stage provides a remote power feed to a terminal by means of a DC voltage, in addition to supplying it with an AC signal. Because the feedback voltage is made independent of the DC remote power feed voltage, the feedback voltage cannot oppose the provision of the remote power feed voltage.